1. Field of the Invention
The present invention relates to a multilayer ceramic substrate which is applicable for use in mobile communication terminals such as portable phones, and also to a method for manufacturing the multilayer ceramic substrate.
2. Description of Related Art
With the need of miniaturizing recently spread mobile communication equipment, such as portable phones, and portable communication terminals, the size reduction and performance increase of high-frequency circuit parts for use therein have been pursued.
In high-frequency circuit substrates, a conventional module including a printed substrate and a capacitor or inductor mounted on its surface has been increasingly replaced by a smaller size module consisting of multilayers of dielectric ceramic substrates each patterned with a capacitor or inductor circuit.
The multilayer ceramic substrate is generally fabricated by providing multilayers of green sheets, such as glass-ceramics, each carrying a predetermined circuit pattern formed by screen printing or the like, and firing the multilayers of green sheets at a temperature of about 900° C. A popular material used for the circuit pattern is silver (Ag) which is highly conductive and air-firable.
However, due to a high tendency of silver to exhibit a migration phenomenon, short circuit defect often occurs between silver wiring electrodes separated by a dielectric layer, which has been a problem.
Japanese Patent Laying-Open No. Hei 11-49531 describes that inclusion of CuO in a glass component of a glass-ceramic for use in a dielectric layer suppresses ionization of Ag during a firing process and thus restrains Ag from diffusing into the glass ceramic.
Japanese Patent Registration No. 2,501,740 discloses a ceramic substrate obtained through low-temperature firing and including anorthite crystals. However, no description is provided as to the relation between the anorthite crystals and the Ag migration.